It's not really confusing at all.

Those two Independents are to the left of mainstream Democrats, so they will almost always vote with the Dems and almost never with the Republicans.

So with the two indepentens who are more reliable voters than some people within the party (Manchin, Sinema) the democrats have 50 votes, and since the senate has 100 members, there would be a 50/50 split, and whenever there is a tie in votes in the senate, the VP breaks the tie, so the democrats have the same effective majority.

If they get 51 then it means the VP won't have to vote, and there can be 1 or 2 members of the dems who don't vote party lines and they can still pass judges/reconciliation/ect.